The present invention is directed to the technology and manufacture of current collectors for electrochemical capacitors and, more particularly, electric double layer (EDL) capacitors. A current collector of the present invention can be used to manufacture electrochemical capacitors having high specific energies and stable energy characteristics.
Electrochemical capacitor current collectors (hereinafter current collectors) have generally been constructed of metals and metal alloys that are stable in specific aqueous and non-aqueous electrolytes. Such metals may include, for example, Al, Ti, Ni, Ag, Nb, Ta, W, Pb and Cu. Notwithstanding such a wide range of metals that may be used in current collectors, many of said materials cannot provide for a wide range of capacitor operating voltages. This is typical, in particular, of capacitors having an aqueous electrolyte. As a result, capacitors employing current collectors of said materials may experience a deterioration of energy and capacity parameters, may have a greater cost of stored energy and, therefore, may be restricted in their application.
The high cost of most of the aforementioned metals is another negative aspect of the use of said metals and their alloys in the manufacture of current collectors. Furthermore, in order to reduce self-discharge, stabilize energy parameters and increase the cycle life of an associated capacitor, high purity versions of said metals are used in current collectors. This impedes development of the technology related to the manufacture of electrochemical capacitors and makes such capacitors difficult to mass produce.
Currently, various activated carbon materials most commonly serve as the active mass of polarizable negative electrochemical capacitor electrodes—whether used with an aqueous or non-aqueous electrolyte. When selecting/manufacturing current collectors for use with electrochemical capacitor electrodes having an activated carbon active mass, the following basic factors are typically taken into consideration: the electrophysical, electric and electrochemical parameters of the current collectors and active material; the operating range of electrode potentials; the properties of the electrolyte used; the operating temperature; the stability of parameters during operation; cycle life; and cost.
Various metals and metal alloys whose surfaces are protected from any negative effects of the electrolyte are often used as current collectors for electrodes having an activated carbon active mass. Application of various electrolyte-stable conductive coatings to the surfaces of a current collector is a commonly used method of protection thereof.
Electrochemical capacitors may also include one or more non-polarizable positive electrodes, such as those made from lead dioxide. Materials commonly used to manufacture a current collector for such a lead dioxide electrode, particularly when said electrode is used with an aqueous sulfuric acid electrolyte, may include for example: (a) lead and its alloys; (b) various alloys of lead with a protective coating; and (c) steel with a protective coating made of graphite foil impregnated by acid resistant varnish. These current collectors may also be used in the manufacture of symmetrical electrochemical capacitors with polarizable carbon electrodes and an aqueous sulfuric acid electrolyte.
A thin layer of material with high specific resistance and unstable electrical parameters will form on work surfaces of lead and lead alloy-based based current collectors after a long period of operation in an aqueous sulfuric acid electrolyte. The use of current collectors with such a layer can cause a degradation in the energy and power parameters, stability of operation, reliability and cycle life of a capacitor.
Thus, in order to ensure that an electrochemical capacitor will have a long service life and highly stable power parameters, there exist stringent requirements with respect to protective coatings for shielding current collectors against degradation from contact with certain electrolytes. On the one hand, it can be understood that it would be difficult to develop a universal protective coating with parameters appropriate to every capacitor. On the other hand, for each specific capacitor (out of a great number of types of these devices) it is generally necessary to develop a special protective coating that is compatible with all the specific properties of the capacitor. This brings about a considerable increase in the cost of an associated current collector, and of the capacitor as a whole. Further, many known protective coatings simply cannot impart a long service life and stable of energy and power parameters to most capacitors and, particularly, to capacitors having aqueous electrolytes.
Current collectors based on steel with graphite foil protective coatings are also known. While these current collectors also have certain drawbacks, the elimination of said drawbacks would make it possible to considerably improve the energy and power parameters of a capacitor and, more importantly, improve its cycle life.
One such known current collector consists of a steel sheet, and a graphite foil protective coating of approximately 0.3 mm thickness that is impregnated by an acid resistant polymer. The protective coating is glued in several spots to a steel basis of the current collector. Following the assembly of a capacitor with this current collector, the capacitor is sealed to ensure that the electrolyte has no contact with the steel basis of the current collector.
The graphite foil that forms the protective coating of this known current collector has a porous structure. In order to prevent infiltration of electrolyte to the surface of the steel basis of the current collector, the pores of the foil are filled with a polymer varnish that is stable in the selected electrolyte. Inasmuch as the protective coating is glued to the steel basis of the current collector in only a few spots, even a single through-pore or micro fracture in the protective coating will be sufficient to allow the electrolyte to gradually penetrate the entire surface of the steel current collector material. The contact of the electrolyte with the steel basis of the current collector will, undoubtedly, bring about dissolution and breakup thereof. During this dissolution and breakup, the transfer to the electrolyte of iron ions and other components of which the steel basis is composed will cause a dramatic increase in the self-discharge current of the capacitor to which the current collector is installed, as well as a decrease in the energy parameters of the capacitor and an accelerated failure thereof.
Other obvious drawbacks of this known current collector include the fact that the graphite foil of the protective coating has a small electric capacity and, when electrolyte gets into its pores, the foil starts to partially perform as an active material in the charge/discharge process of the capacitor. Over a long period of operation, this process brings about swelling, deterioration of mechanical parameters, and a partial or total breakup of the graphite foil structure. The result is an increase in the electric resistance of the current collector and of the capacitor as a whole.
It should also be noted that when impregnating the graphite foil of the protective coating of this known current collector with a non-conducting polymer, the polymer makes contact with the carbon particles of the foil and increases its electric resistance. This also increases the electric resistance of the current collector and of an associated capacitor as a whole.
The particular design of the current collector itself is another drawback of this known current collector. That is, this known current collector is designed for use in a capacitor having one positive electrode plate and two negative electrode plates. Consequently, this known current collector is not amenable to use in a capacitor with a different number of positive and/or negative electrode plates connected in parallel. Therefore, this current collector cannot be used to create capacitors of high electric capacity and acceptable energy and power parameters. The use of parallel and series cell connection in order to obtain a capacitor module with high stored energy will actually bring about a significant reduction in the specific energy and power parameters of a capacitor that has only one positive electrode plate. Therefore, it can be understood that such an electrochemical capacitor employing this known current collector would have a low specific energy, low reliability, unstable energy parameters, a high energy storage cost, and a short service life. The low specific parameters of such a capacitor would significantly limit the scope of its application.
It is known that the contact resistance between the active material of an electrode and its current collector plays an important role in ensuring that an electrochemical capacitor exhibits stable energy and power parameters. The electric resistance between the materials of the electrode and its current collector are directly dependent on the electrophysical parameters of the materials and the electrolyte used. Electrons are transferred from the active mass of the electrode to the current collector and/or from the current collector to the active mass of the electrode during the charging and discharge of a capacitor. Consequently, in order to obtain high power and stable parameters of the capacitor, it is necessary to ensure a minimum height of the energy barrier of the electric charge transfer and to ensure that it does not change during capacitor operation.
The active materials (i.e., activated carbon powders) that are typically used for the manufacture of polarizable electrochemical capacitor electrodes are mainly degenerate p-type semiconductors, whose Fermi level (EF) is in the valence band. During the charge and discharge of capacitors having such electrodes, there occur changes in charge carrier concentration in the near-surface layer of the pore walls of the active mass, as well as in the area of contact between the active mass of the electrode and the current collector. This causes a change in the conductivity value of the active mass, and the rate of such change depends on the depth of charge and discharge of the capacitor. The conductivity of electrodes of capacitors having high specific electric capacity during their charge and discharge changes in a wide range.
As can be observed in FIG. 1, during high polarization of a capacitor's electrode (in order to obtain high voltage and energy), there occurs a change of the type of conductivity present in the surface layers of the electrode. This change occurs in the area of contact between the active mass 1 of the electrode and the current collector 2, from the side of the active mass, and in the near-surface layers 3 of the walls of its pores 5. This figure shows that during significant distortion of the zones in the area of contact having δ thickness (and in the near-surface layers of the walls of the pores of the active mass), the Fermi level Ef is above the bottom of the conductivity zone. This implies that the material in this area is a degenerate material of p-type conductivity. This brings about the occurrence of a p-n junction in the contact area along the side of the active mass. The thickness and distribution of the volume of the spatial charge of the p-n junction depends on the electrophysical parameters of the solid electrode material, the electrolyte 4, and the potential of the electrode.